1. Technical Field
Embodiments of the invention relate to photocoupler output signal receiving circuits used for isolating and transmitting signals in an intelligent power module (hereafter abbreviated to IPM).
2. Related Art
PWM (Pulse Width Modulation) inverters are widely used as power conversion devices that drive a three phase alternating current motor. Also, PWM converters are widely used as power conversion devices that obtain direct current voltage from a three phase alternating current power source. These power conversion devices are configured of a power circuit that carries out a direct current to alternating current or alternating current to direct current power conversion, a pre-driver that controls and drives the power circuit, a protective circuit, and a control circuit that centrally controls the circuits.
The power circuit is a circuit that supplies a drive current to the three phase alternating current motor and generates a main direct current power source, and is configured of a switching element such as an IGBT (insulated gate bipolar transistor), and a free wheel diode for causing energy when the switching element is turned off to flow as current. Also, the pre-driver is a circuit for driving the switching element. Furthermore, the protective circuit is a circuit that has various kinds of protective function, such as overcurrent protection, short circuit protection, overheat protection, and current and voltage drop protection.
The control circuit is configured of a controller and a central processing unit (CPU/ROM) including a memory such as a ROM (read only memory), carries out processes such as generating a PWM signal and issuing an alarm when detecting a problem, and centrally controls the PWM inverter or PWM converter.
Recently, a semiconductor device called an IPM in which, of the configurations, the power circuit, drive circuit, and protective circuit are incorporated as one package, has become commercially available, and is being widely used.
Herein, FIG. 7 is a block diagram showing a circuit configuration of an inverter device into which an IPM is incorporated.
The inverter device is connected to a two phase or three phase alternating current power source, and includes a converter 1 that converts alternating current into direct current, an electrolytic capacitor 2 for smoothing, an IPM 3, a buffer 4, a controller 5, a central processing unit (CPU/ROM) 6 including a memory, a switching transistor 7, a power source circuit formed of transformers 8 and 9 and a switching regulator 10, and a current transformer CT connected between the output side of the IPM 3 and a three phase alternating current motor M.
The controller 5 and central processing unit (CPU/ROM) 6 will be collectively called a control circuit 24.
The IPM 3 is integrally configured of a three phase inverter 11, configured of a semiconductor switching element such as an IGBT and a free wheel diode, whose alternating current output side is connected to the motor M, a pre-driver 12 that controls and drives the inverter 11, a protective circuit 13, an overcurrent detection sensor 14 and overheat detection sensor 15 connected to the protective circuit 13, a brake power element 16 and resistor 17 used when controlling the motor M so as to decelerate, and a pre-driver 18 that controls and drives the brake power element 16. Herein, a depiction of a signal line that sends a drive signal from the pre-driver 12 to the inverter 11 is omitted.
A control signal to the IPM 3 is sent from the control circuit 24 via the buffer 4 and photocouplers 21 and 23 to the pre-drivers 12 and 18 respectively, while an alarm signal when an overcurrent condition or overheat condition is detected by the sensor 14 or 15 is sent from the protective circuit 13 via the photocoupler 22 to the buffer 4.
Also, an output signal of the current transformer CT is input into the controller 5. As the current transformer CT carries out various kinds of control by detecting the output current of the IPM 3 and feeding back to the controller 5, it includes three through holes, and is installed inside the inverter device in a condition wherein wires or bars, which are the three phases of output current line of the inverter 11, are inserted into the through holes.
The inverter device with the heretofore described configuration is such that a direct current voltage converted by the converter 1 is converted into a three phase alternating current voltage by the inverter 11, and supplied to the motor M. The inverter 11 has a bridge circuit formed of a switching element and a free wheel diode, wherein the switching element converts a current eventually caused to flow through the motor M into an alternating current by carrying out a chopping control on the direct current voltage, and controls the motor M at variable speeds by changing the frequency of the alternating current.
Also, the controller 5 controls so that no distortion occurs in an inverter 11 output current waveform detected by the current transformer CT, and controls so that the output current does not reach or exceed a predetermined value.
The pre-drivers 12 and 18, which receive a control signal from the control circuit 24 after it is isolated by the photocouplers 21 and 23, include a current discharge function and a current intake function with respect to the photocouplers.
FIG. 8 is a block diagram of an input circuit of, for example, the pre-driver 12, with the input circuit of the pre-driver 18 also being configured in the same way as in FIG. 8. In FIG. 8, 25 indicates a direct current power source (a power source voltage Vcc is, for example, 15V), 121 inside the pre-driver 12 is a constant current circuit (discharge side), 122 is a constant current circuit (intake side), 123 is a power source terminal to which the power source voltage Vcc is applied, 124 is an input terminal to which an output signal Vin of the photocoupler 21 is applied, 125 is a ground terminal, Rpu is a pull-up resistor, ZD is a Zener diode, and SW1 and SW2 are switches (semiconductor switching elements) that operate in a mutually complementary way. 21a inside the photocoupler 21 is a light emitting diode acting as a light emitting unit, and 21b is a phototransistor acting as a light receiving unit.
The output signal Vin will hereafter be called an “input signal Vin” when seen from the pre-driver 12 side.
Herein, the current discharge function and current intake function of the pre-driver 12 have an object of improving noise resistance.
That is, as shown in FIG. 9, while the voltage level of the input signal Vin in FIG. 8 exceeds a threshold value VinL (before a time t1), a current IinH flows as a discharge current from the input terminal 124 to the exterior via the constant current circuit 121 and switch SW1 of FIG. 8, and the photocoupler 21 will not reach the threshold value VinL unless it draws in more current.
Meanwhile, when the voltage level of the input signal Vin drops below the threshold value VinL at the time t1, the pre-driver 12 stops the discharge of the current IinH. Then, until the voltage level of the input signal Vin exceeds a threshold value VinH at a time t2, a current IinL flows as an intake current from the input terminal 124 to the interior (times t1 to t2) via the constant current circuit 122 and switch SW2 of FIG. 8, and the photocoupler 21 will not reach the threshold value VinH unless it draws in more current.
That is, the pre-driver 12 carries out a discharge or intake of current in accordance with the voltage level of the input signal Vin in order to improve the noise resistance of the photocoupler 21, with a maximum of the discharge current IinH of FIG. 9 flowing through the collector of the phototransistor 21b. 
An isolating type signal transmission circuit, wherein a drive signal to an IPM switching element is isolated and transmitted by a photocoupler, is described in Japanese Patent Publication No. JP-A-2007-150003 (also referred to herein as “Patent Document 1”). The signal transmission circuit is such that, in order to prevent a drop in current conversion efficiency due to temporal depreciation of the photocoupler when routinely increasing the intake current, or the like, a transient current with an amplitude larger than normal is caused to flow through the light emitting diode when a switching element connected in series to the light emitting diode of the photocoupler is off, and in the same way, when a transistor inside a constant current circuit connected in series to the light emitting diode is maintained in a saturated condition, and the switching element is turned on.
Also, there is disclosed in Japanese Patent Publication No. JP-A-2008-172513 (also referred to herein as “Patent Document 2”) an invention that, with an object of increasing photocoupler lifespan, includes a current compensation circuit that detects a temporal depreciation signal having a correlation with characteristic depreciation of a photocoupler light emitting diode, and increases the on-state current of the light emitting diode in accordance with the condition thereof.
As is also described in Patent Documents 1 and 2, the photocoupler characteristics depreciate with the passing of time, and when the internal light emitting diode is used for a long time, the light emitting efficiency decreases, and a current conversion efficiency CTR (=Ic/If) drops. Herein, If is the input current flowing through the light emitting diode, and Ic is the output current (collector current) of the phototransistor on the output side.
When actually using the photocoupler, the estimated lifespan of the photocoupler is calculated from the current conversion efficiency CTR, the load design values, and the like, and If is determined. In order to increase the lifespan of the photocoupler, it is necessary to suppress the size of If. However, when If is too small, noise resistance drops, and operating speed drops.
FIG. 10 shows Vce (the voltage between the collector and emitter of the phototransistor)—Ic (the phototransistor collector current) characteristics, with the current If flowing through the photocoupler light emitting diode as a parameter.
As previously described, in order to increase the phototransistor collector current Ic to improve the pre-driver noise resistance, it is necessary to increase the current If flowing through the light emitting diode.
Meanwhile, FIG. 11 shows lifespan depreciation characteristics of the photocoupler current conversion efficiency CTR, wherein the tendency is that the more the current If increases, the shorter the time in which the current conversion efficiency CTR drops. That is, as the current conversion efficiency CTR drops in a short time when routinely increasing the phototransistor collector current, in other words, the light emitting diode current If, in order to improve noise resistance, the current drawing capability of the photocoupler after the photocoupler is turned on drops. Because of this, a long time is needed until the input signal voltage level drops below the threshold value VinL of FIG. 9, as a result of which, a PWM signal for driving the switching element is not accurately transmitted from the pre-driver, causing malfunction.
Consequently, it cannot be said that the method whereby the light emitting diode current If is increased, as in the heretofore known technology according to Patent Document 1 or Patent Document 2, is optimal for preventing a drop in the current conversion efficiency CTR or preventing malfunction. Thus, there are needs in the art for an improved a photocoupler output signal receiving circuit.